diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 15276e7259f3..9c18c17a25bf 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -169,6 +169,11 @@ Tester.prototype = {
         }
       };
 
+      // Clear document.popupNode.  The test could have set it to a custom value
+      // for its own purposes, nulling it out it will go back to the default
+      // behavior of returning the last opened popup.
+      document.popupNode = null;
+
       // Note the test run time
       let time = Date.now() - this.lastStartTime;
       this.dumper.dump("INFO TEST-END | " + this.currentTest.path + " | finished in " + time + "ms\n");
diff --git a/testing/mochitest/tests/browser/Makefile.in b/testing/mochitest/tests/browser/Makefile.in
index e82430e96c24..d1ecdb722873 100644
--- a/testing/mochitest/tests/browser/Makefile.in
+++ b/testing/mochitest/tests/browser/Makefile.in
@@ -50,6 +50,8 @@ _BROWSER_TEST_FILES = \
 	                  browser_pass.js \
 	                  browser_async.js \
 	                  browser_privileges.js \
+	                  browser_popupNode.js \
+	                  browser_popupNode_check.js \
 # Disabled, these are only good for testing the harness' failure reporting
 #	                  browser_zz_fail_openwindow.js \
 #	                  browser_fail.js \
diff --git a/testing/mochitest/tests/browser/browser_popupNode.js b/testing/mochitest/tests/browser/browser_popupNode.js
new file mode 100644
index 000000000000..c6042011cdf5
--- /dev/null
+++ b/testing/mochitest/tests/browser/browser_popupNode.js
@@ -0,0 +1,4 @@
+function test() {
+  document.popupNode = document;
+  isnot(document.popupNode, null, "document.popupNode has been correctly set");
+}
diff --git a/testing/mochitest/tests/browser/browser_popupNode_check.js b/testing/mochitest/tests/browser/browser_popupNode_check.js
new file mode 100644
index 000000000000..fb85378d9d37
--- /dev/null
+++ b/testing/mochitest/tests/browser/browser_popupNode_check.js
@@ -0,0 +1,3 @@
+function test() {
+  is(document.popupNode, null, "document.popupNode has been correctly cleared");
+}